U.S. patent number 6,614,338 [Application Number 09/805,647] was granted by the patent office on 2003-09-02 for inductor and method for manufacturing same.
This patent grant is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Junichi Hamatani, Hisato Oshima.
United States Patent |
6,614,338 |
Hamatani , et al. |
September 2, 2003 |
Inductor and method for manufacturing same
Abstract
A inductor is constructed such that at least about two-thirds of
a final winding of wire at each end of an internal conductor-coil
embedded in a molded magnetic body project from end surfaces of the
molded magnetic body by at least about one-fifth of the diameter of
the wire. External electrodes are connected with respective
portions of the internal conductor-coil, which are exposed at the
respective end surfaces of the molded magnetic body.
Inventors: |
Hamatani; Junichi (Shiga-ken,
JP), Oshima; Hisato (Takefu, JP) |
Assignee: |
Murata Manufacturing Co., Ltd.
(Kyoto, JP)
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Family
ID: |
18589324 |
Appl.
No.: |
09/805,647 |
Filed: |
March 14, 2001 |
Foreign Application Priority Data
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Mar 14, 2000 [JP] |
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2000-070614 |
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Current U.S.
Class: |
336/83; 336/192;
336/200; 336/223 |
Current CPC
Class: |
H01F
27/292 (20130101); H01F 17/045 (20130101); H01F
41/046 (20130101); Y10T 29/4902 (20150115); Y10T
29/49073 (20150115) |
Current International
Class: |
H01F
41/04 (20060101); H01F 27/29 (20060101); H01F
17/04 (20060101); H01F 005/00 () |
Field of
Search: |
;336/83,200,223,192,65,96 ;29/602.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2274165 |
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Dec 1999 |
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CA |
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0 984 465 |
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Mar 2000 |
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EP |
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8-306537 |
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Nov 1996 |
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JP |
|
Primary Examiner: Mai; Ann
Attorney, Agent or Firm: Keating & Bennett, LLP
Claims
What is claimed is:
1. An inductor comprising: a molded magnetic body including a
molded magnetic material member including a powdered magnetic
material and a resin-based material; an internal conductor-coil
embedded in the molded magnetic body such that both ends of the
internal conductor-coil are exposed from both end surfaces of the
molded magnetic body, respectively; and a pair of external
electrodes provided at the respective end surfaces of the molded
magnetic body to be connected to the internal conductor-coil at the
respective ends thereof; wherein at least about two thirds of a
final winding of wire at the respective ends of the internal
conductor-coil project from the end surface of the molded magnetic
body by at least about one fifth of the diameter of the wire of the
internal conductor-coil; and the external electrodes are each
connected with at least about two thirds of the final winding of
wire at the respective ends of the internal conductor-coil, which
project from the end surface of the molded magnetic body by at
least about one fifth of the diameter of the wire of the internal
conductor-coil.
2. An inductor according to claim 1, wherein the center of the
final winding of wire at the respective ends of the internal
conductor-coil is spaced away from the center of each end surface
of the molded magnetic body by a distance not greater than about
half of the inner diameter of the internal conductor-coil.
3. An inductor according to claim 1, wherein the external
electrodes are each defined by a plurality of layers of metallic
films.
4. An inductor according to claim 3, wherein the center of the
final winding of wire at the respective ends of the internal
conductor-coil is spaced away from the center of each end surface
of the molded magnetic body by a distance not greater than about
half of the inner diameter of the internal conductor-coil.
5. An inductor according to claim 1, wherein said molded magnetic
body is made of a ferrite resin.
6. An inductor according to claim 5, wherein said ferrite resin
includes a polyphenylene sulfite resin and a powdered ferrite
including iron oxide, nickel oxide, copper oxide and zinc
oxide.
7. An inductor according to claim 1, wherein said internal
conductor-coil is defined by a copper wire.
8. An inductor according to claim 7, wherein said copper wire
defining said internal conductor-coil has an outer diameter of
about 0.2 mm, a length of about 3.2 mm, and an inner diameter of
about 1.8 mm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates inductors and a method for
manufacturing inductors. In particular, the present invention
relates to an inductor and a method for manufacturing inductors, in
which a molded magnetic body provided with a pair of external
electrodes connected to an internal conductor-coil embedded in the
molded magnetic body is made by molding a magnetic material which
includes a powdered magnetic material and a resin.
2. Description of the Related Art
A conventional surface-mounting-type inductor 60 shown in FIG. 12
includes a coil (internal conductor-coil) 52 defining an inductance
element is embedded in a molded magnetic body 53 formed by molding
a magnetic material 51 including a powdered magnetic material and a
resin. The molded magnetic body 53 is provided at ends thereof with
a pair of external electrodes 54a and 54b connected to the coil 52
at ends 52a and 52b, respectively, of the coil 52.
The inductor 60 is manufactured, for example, such that a coil (an
air-core coil) formed by densely winding an insulative covered
copper wire and cutting the same by a predetermined length is
provided in a mold, a magnetic molding compound made by kneading a
powdered magnetic material and a resin is injected into the mold
and is provided around the coil (inside and outside the coil), and
the mold is released, thereby producing a molded magnetic body. The
molded magnetic body is provided with external electrodes made of
metallic films at ends of the molded magnetic body including
exposed portions of the coil, the external electrodes are formed by
coating, baking, deposition, or sputtering of a conductive paste,
such that the external electrodes are connected to the exposed
portions of the coil.
The inductor 60 can be manufactured only by forming the molded
magnetic body 53 by molding the magnetic material 51 which is made
by kneading a powdered magnetic material and a resin, and providing
the external electrodes 54a and 54b made of metallic films.
Therefore, a firing process at a high temperature and a baking
process for the electrodes, which are necessary in manufacturing a
conventional ceramic inductor including a magnetic ceramic, are not
necessary, whereby manufacturing costs are reduced.
In the inductor 60, the external electrodes 54a and 54b are
arranged to connect to exposed portions 52a and 52b which are
portions of final windings of wire of a coil 52. The shape and the
position (for example, the position in a vertical direction of the
exposed portion 52a or 52b) of the exposed portion 52a or 52b of
the coil 52 often differ according to each inductor 60 due to
deformation of the coil 52 during injection of the magnetic
material 51.
In a conventional manufacturing method, since the coil 52 is
deformed due to being pressed by the mold when the length of the
coil 52 is greater than that of the molded magnetic body 53, the
length of the coil 52 must be substantially the same as that of the
molded magnetic body 53. Therefore, as shown in FIG. 13, the
exposed portion 52a or 52b of the coil 52 is formed partially in
the final winding of wire of the coil 52 at the end of the molded
magnetic body 53, and the area of the exposed portion 52a or 52b is
reduced because of the difficulty in forming the exposed portion
52a or 52b which significantly protrudes from the end of the molded
magnetic body 53.
Therefore, the connection between the coil 52 and the external
electrodes 54a and 54b is not secure and an overcurrent is applied
to the coil.
In another conventional inductor, the inductor 60 is provided with
the external electrodes 54a and 54b which are defined by a
plurality of layers such that the external electrodes 54a and 54b
are easily soldered, a metallic film, such as solder, tin, or
silver, to which solder easily adheres, being used as an outermost
layer. When the inductor 60 is mounted on a mounting body such as a
printed circuit board 61 via a method such as reflow-soldering, as
shown in FIG. 14, a solder fillet 62 is raised to a height Hs which
is at least 1/3 of a height H of the inductor 60 because the solder
easily adheres to the external electrodes 54a and 54b. The inductor
60 is mounted such that the solder fillet 62 is electrically
connected to the external electrodes 54a and 54b.
In the conventional method of manufacturing an inductor, a magnetic
molding compound is injected into the mold in which a coil is not
firmly affixed in a desired position in the mold. Therefore, there
is a risk that the coil will move depending on the direction of
flow of the magnetic molding compound during the injection
process.
For example, when the inductor 60 in which the coil 52 is
displaced, as shown in FIG. 15, is mounted on the printed circuit
board 61, the solder fillet 62 does not reach the positions of the
exposed portions 52a and 52b of the coil 52 with the external
electrodes 54a and 54b therebetween even when the solder fillet 62
is raised to the height Hs which is at least 1/3 of the height H of
the inductor 60, because the exposed portions 52a and 52b of the
coil 52 are excessively elevated, and a gap G is produced between a
lower end of the exposed portion 52a or 52b and an upper end of the
solder fillet 62. The current applied to the inductor 60 flows
through only the external electrodes 54a and 54b at the gap
portion. Therefore, when the external electrodes 54a and 54b are
made of a metallic thin film such as a solder film, long-term
reliability and unsafe operation when an overcurrent is applied
occur, due to insufficient current capacity in the portion having
the gap.
To overcome these problems, the thickness of the metallic film
defining the external electrodes 54a and 54b may be increased.
However, the manufacturing costs also increase with the increased
thickness of the film.
The external electrodes 54a and 54b may be formed by bonding
metallic plates to the ends of the molded magnetic body 53, each of
the metallic plates having a sufficient thickness required for the
current capacity. However, the manufacturing costs are also
increased with this method.
SUMMARY OF THE INVENTION
To overcome the above-described problems with the prior art,
preferred embodiments of the present invention provide an inductor
and a method for manufacturing the inductor, in which reliable
connection between an internal conductor-coil and external
electrodes, long-term reliability after mounted, and safety when
applied with an overcurrent are achieved.
An inductor according to a preferred embodiment of the present
invention includes a molded magnetic body formed by molding a
magnetic material including a powdered magnetic material and a
resin-based material, an internal conductor-coil embedded in the
molded magnetic body such that both ends of the internal
conductor-coil are exposed from both end surfaces of the molded
magnetic body, respectively, and a pair of external electrodes
provided at the respective end surfaces of the molded magnetic body
to connect to the internal conductor-coil at the respective ends
thereof. At least two thirds of a final winding of wire at each of
the ends of the internal conductor-coil project from the end
surface of the molded magnetic body by at least about one fifth of
the diameter of the wire of the internal conductor-coil. The
external electrodes are each connected with at least about two
thirds of the final winding of wire at each of the ends of the
internal conductor-coil, which project from the end surface of the
molded magnetic body by at least about one fifth of the diameter of
the wire of the internal conductor-coil.
At least about 2/3 of a final winding of wire at each end of the
internal conductor-coil project from the end surface of the molded
magnetic body by an amount of at least about 1/5 of the diameter of
a wire, and the external electrodes are each connected with at
least about 2/3 of the final winding of wire at each of the ends of
the internal conductor-coil, which project from the end surface of
the molded magnetic body by at least about 1/5 of the diameter of
the wire of the internal conductor-coil, whereby reliable
connection is established by increasing the area of connection
between the internal conductor-coil and the external electrodes,
and long-term reliability after mounted and safety when applied
with an overcurrent are greatly improved. Moreover, the thickness
of the external electrodes is greatly reduced, thereby greatly
reducing the manufacturing costs.
The resin-based material used together with the powdered magnetic
material, according to various preferred embodiments of the present
invention, includes various materials, such as an epoxy resin, a
synthetic resin including polyphenylene sulfide, and a rubber resin
including a chloroprene rubber or a silicone rubber.
The external electrodes are preferably defined by a plurality of
layers of metallic films.
When each external electrode is defined by a plurality of layers,
an inductor having reliable electrical connection and solderability
is provided by depositing a tin-plating film or a solder-plating
film on a base metallic film defining the external electrodes.
The center of the final winding of wire at each of the ends of the
internal conductor-coil is spaced away from the center of each end
surface of the molded magnetic body by a distance not greater than
about half of the inner diameter of the internal
conductor-coil.
Since the center of the final winding of wire at each of the ends
of the internal conductor-coil is spaced away from the center of
each end surface of the molded magnetic body by a distance not
greater than about 1/2 of the inner diameter of the internal
conductor-coil, the condition described below is efficiently
prevented from occurring. That is, when an inductor in which an
internal conductor-coil is displaced is mounted on a printed
circuit board, a solder fillet does not reach a position where the
solder fillet is opposed to an exposed portion of the coil with
external electrodes therebetween because the position of the
exposed portion of the internal conductor-coil is excessively
elevated, and a gap is produced between a lower end of the exposed
portion and an upper end of the solder fillet. Therefore, when the
external electrodes are made of a metallic thin film such as a
plating film, reliability and safety are substantially diminished
when an overcurrent is applied, due to insufficient current
capacity in the portion corresponding to the gap. These problems
are prevented by preferred embodiments of the present
invention.
According to another preferred embodiment of the present invention,
a method for manufacturing an inductor includes the steps of
preparing the internal conductor-coil, setting the internal
conductor-coil in a mold, coupling the internal conductor-coil with
a coil-supporting member at an inner periphery of the internal
conductor-coil for supporting the internal conductor-coil at the
inner periphery thereof, thereby preventing the internal
conductor-coil from being deformed and maintaining the internal
conductor-coil in a position and shape in which the internal
conductor-coil is disposed to be exposed from a magnetic material
at ends of the internal conductor-coil, and a first injection step
of injecting the magnetic material through a gate provided at a
predetermined position of the mold into a region of the mold except
for a region at the inner periphery of the internal conductor-coil
in which the coil-supporting member is disposed, removing the
coil-supporting member after the magnetic material injected in the
first injection step cures, and a second injection step of
injecting the magnetic material into the region at the inner
periphery of the internal conductor-coil through another gate
provided at a predetermined position of the mold, thereby forming a
molded magnetic body in which a major portion of the internal
conductor-coil is embedded in the molded magnetic body and at least
about two thirds of a final winding of wire at each end of the
internal conductor-coil project from an end surface of the molded
magnetic body by at least about one fifth of the diameter of the
wire of the internal conductor-coil, and forming a pair of external
electrodes at the respective end surfaces of the molded magnetic
body so that the external electrodes are each connected with at
least about two thirds of the final winding of wire at each of the
ends of the internal conductor-coil, which project from the end
surface of the molded magnetic body by at least about one fifth of
the diameter of the wire of the internal conductor-coil.
The internal conductor-coil is supported by the coil-supporting
member at the inner periphery of the internal conductor-coil so as
to prevent the internal conductor-coil from being deformed and to
maintain the internal conductor-coil in a position and a shape in
which the internal conductor-coil is disposed so as to be exposed
from a magnetic material at ends of the internal conductor-coil,
the magnetic material is injected into a region of the mold except
for a region at the inner periphery of the internal conductor-coil,
the coil-supporting member is removed after the magnetic material
cures, and the magnetic material is injected into the region at the
inner periphery of the internal conductor-coil, thereby forming a
molded magnetic body in which at least about 2/3 of a final winding
of wire at each end of the internal conductor-coil project from an
end face of the molded magnetic body by at least about 1/5 of the
diameter of a wire of the internal conductor-coil. A pair of
external electrodes are provided at the respective end surfaces of
the molded magnetic body such that the external electrodes are each
connected with at least about 2/3 of the final winding of wire at
each of the ends of the internal conductor-coil, which project from
the end surface of the molded magnetic body by at least about 1/5
of the diameter of the wire of the internal conductor-coil. Thus,
the inductor according to preferred embodiments of the present
invention is efficiently and reliably manufactured.
The mold is provided with substantially annular concave portions,
each of the annular concave portions is provided at an inner
surface of the mold opposing the end of the internal conductor-coil
such that at least one portion of the final winding of wire at the
end of the internal conductor-coil is fitted with the annular
concave portion.
By using the mold which is provided with substantially annular
concave portions, each of the annular concave portions at an inner
surface of the mold opposing the end of the internal conductor-coil
and at least one portion of the final winding of wire at the end of
the internal conductor-coil is fitted with the annular concave
portion, a molded magnetic body, in which at least about 2/3 of a
final winding of wire at each end of the internal conductor-coil
project from an end surface of the molded magnetic body by at least
about 1/5 of the diameter of the wire of the internal
conductor-coil, is reliably produced.
The center of each substantially annular concave portion provided
at the inner surface of the mold and the center of each end surface
of the molded magnetic body substantially correspond to each
other.
When the centers of each substantially annular concave portion
provided at the inner surface of the mold and each end surface of
the molded magnetic body substantially coincide with each other, a
risk of a phenomena described below is efficiently avoided. That
is, when an inductor in which an internal conductor-coil is
displaced is mounted on a printed circuit board, a solder fillet
does not reach a position where the solder fillet is opposed to an
exposed portion of the coil with external electrodes therebetween
because the position of the exposed portion of the internal
conductor-coil is excessively elevated, and a gap is produced
between a lower end of the exposed portion and an upper end of the
solder fillet. Therefore, when the external electrodes are made of
a metallic thin film such as a plating film, long-term reliability
and safety is substantially diminished when an overcurrent is
applied, due to insufficient current capacity in the portion
corresponding to the gap. These problems are prevented by the
preferred embodiments of the present invention.
Other features, elements, characteristics and advantages of the
present invention will become apparent from the detailed
description of preferred embodiments thereof with reference to the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an inductor according to a preferred
embodiment of the present invention.
FIG. 2 is a side view of the inductor according to the preferred
embodiment of the present invention shown in FIG. 1.
FIGS. 3A and 3B are schematic plan view and side view,
respectively, of the mounted inductor according to a preferred
embodiment of the present invention.
FIG. 4 is a graph showing the relationship between the projection
amount of an internal conductor-coil (ratio to the diameter of a
wire) and the temperature rise in a connection portion of external
electrodes and the internal conductor-coil.
FIG. 5 is a graph showing the relationship between the area of
exposed portions of the internal conductor-coil (ratio of the
length of the exposed portions of the internal conductor-coil to a
winding of wire) and the temperature rise in the connection part of
the external electrodes and the internal conductor-coil.
FIG. 6 is a graph showing the relationship between the amount of
offset of the internal conductor-coil (ratio to the inner diameter
of the internal conductor-coil) and the temperature rise in the
external electrodes.
FIG. 7 is a sectional view of a mold to be used in a method for
manufacturing an inductor, according to another preferred
embodiment of the present invention.
FIG. 8 is a sectional view of the mold in which the internal
conductor-coil is set in a process of the method for manufacturing
an inductor, according to a preferred embodiment of the present
invention.
FIG. 9 is a sectional view showing a first step of injection of the
method for manufacturing an inductor, according to a preferred
embodiment of the present invention.
FIG. 10 is a sectional view of the mold from which a
coil-supporting member has been removed after the first step of
injection of the method for manufacturing an inductor, according to
a preferred embodiment of the present invention.
FIG. 11 is sectional view showing a second step of injection of the
method for manufacturing an inductor, according to a preferred
embodiment of the present invention.
FIG. 12 is a sectional view of a conventional inductor.
FIG. 13 is a side view of the conventional inductor.
FIG. 14 is a front view of the mounted conventional inductor.
FIG. 15 is a side view of the mounted conventional inductor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments according to the present invention are
described in detail.
FIG. 1 is a sectional view of an inductor according to a preferred
embodiment of the present invention. FIG. 2 is a side view of the
inductor.
An inductor 10 according to the present preferred embodiment shown
in FIGS. 1 and 2 preferably includes a molded magnetic body
(magnetic core) 3 formed by molding a magnetic material 1 including
a powdered magnetic material and a resin kneaded with each other in
a desired shape, an internal conductor-coil 2, defining an
inductance element, embedded in the molded magnetic body 3 and
exposed at ends 2a and 2b of the internal conductor-coil 2 from end
surfaces 3a and 3b of the molded magnetic body 3, and a pair of
external electrodes 4a and 4b provided at the end surfaces 3a and
3b, respectively, of the molded magnetic body 3 connected to the
internal conductor-coil 2 at the ends 2a and 2b thereof,
respectively. The dimensions of the inductor 10 are, for example,
approximately 4.5 mm.times.3.2 mm.times.3.2 mm.
The molded magnetic body (magnetic core) 3 is preferably made of a
ferrite resin which is formed by kneading a PPS (polyphenylene
sulfide) resin and a powdered ferrite including iron oxide
(Fe.sub.2 O.sub.3), nickel oxide (NiO), copper oxide (CuO), and
zinc oxide (ZnO).
The internal conductor-coil 2 is formed by winding a copper wire
having a diameter of, for example, about 0.2 mm, and has a length
of, for example, about 3.2 mm and an inner diameter of, for
example, about 1.8 mm.
In the inductor 10 shown in FIG. 1, a significant portion of a
final winding of wire at each end of the internal conductor-coil 2
is exposed, and a major portion of each of the ends (exposed
portion) 2a and 2b projects along the axis of the internal
conductor-coil 2 such that a projection amount L from the end
surface 3a or 3b of the molded magnetic body 3 is at least about
1/5 of diameter D of the wire.
The external electrodes 4a and 4b extend from the end surfaces 3a
and 3b of the molded magnetic body 3 to peripheral surfaces (side
surfaces) thereof and connected to the exposed portions 2a and 2b
of the internal conductor-coil 2. The external electrodes 4a and 4b
are each preferably defined by a plurality of layers including a
nickel-plating film electrically connected to the internal
conductor-coil 2 and a tin-plating film which is provided on the
nickel-plating film to improve solderability.
The inductor 10 is arranged so that a center X of a final winding
of wire at each end of the internal conductor-coil 2 is positioned
away from a center Y of each end surface 3a or 3b of the molded
magnetic body 3 by a distance not greater than about 1/2 of the
inner diameter of the internal conductor-coil 2 (see FIG. 2). That
is, the amount of offset of the center X of the final winding of
wire at each end of the internal conductor-coil 2 is not greater
than about 1/2 of the inner diameter of the internal conductor-coil
2 from the center Y of each end surface 3a or 3b of the molded
magnetic body 3.
In the inductor 10 thus formed, a significant portion (at least
about 2/3 windings) of the final winding of wire at each end of the
inductor coil 2 projects substantially in the axial direction of
the internal conductor-coil 2 from the end surface 3a or 3b of the
molded magnetic body 3 by at least about 1/5 of the diameter of the
wire, and the external electrodes 4a and 4b are disposed to be
connected to the exposed portions 2a and 2b at the ends of the
internal conductor-coil 2. Therefore, contact areas between the
internal conductor-coil 2 and the respective external electrodes 4a
and 4b are greatly increased, and electrical current is reliably
applied to the connection portion between the external electrodes
4a and 4b and the internal conductor-coil 2, whereby the long-term
reliability after mounted and the safety when an overcurrent is
applied is ensured.
Since the center X of the final winding of wire at each end of the
internal conductor-coil 2 of the inductor 10 is positioned at a
distance not greater than about 1/2 of the inner diameter of the
internal conductor-coil 2 from the center Y of the end surface 3a
or 3b of the molded magnetic body 3, a solder fillet 12 is raised
to a position corresponding to the exposed portion 2a (2b) of the
internal conductor-coil 2 via the external electrode 4a (4b), that
is, a height (position) Hs of the upper end of the solder fillet 12
is greater (higher) than a height (position) He of the lower end of
the exposed portion 2a (2b) of the internal conductor-coil 2.
Therefore, a gap between the lower end of the exposed portion 2a
(2b) of the internal conductor-coil 2 and the upper end of the
solder fillet 12 is not produced. Even when the external electrodes
4a and 4b are made with a metallic thin film such as a plated film,
the long-term reliability is ensured by maintaining current
capacity of these portions and the safety when an overcurrent is
applied is efficiently maintained.
FIG. 4 is a graph showing the relationship between projection
amount of the internal conductor-coil 2 (ratio to the diameter of
the wire) from the end surface 3a or 3b of the molded magnetic body
3 and temperature rise in the connected portions between the
external electrodes 4a and 4b and the internal conductor-coil 2,
when an electrical current of 2 amperes is applied.
FIG. 4 shows that the temperature rise in the connected portions is
greatly suppressed when the projection amount (ratio to the
diameter of the wire) of the internal conductor-coil 2 from the end
surface 3a or 3b of the molded magnetic body 3 is at least about
1/5 (approximately 0.04 mm) of the diameter D (approximately 0.2
mm) of the wire.
Generally, an inductor including a coil wire having a large
diameter has a large rated current. Particularly, the temperature
rise in the connected portions is suppressed by setting the
projection amount (ratio to the diameter of the coil wire) of the
internal conductor-coil 2 from the end surface 3a or 3b of the
molded magnetic body 3 to at least about 1/5 of the diameter D of
the wire, thereby greatly improving reliability.
FIG. 5 is a graph showing the relationship between the ratio of the
exposed portion per winding of wire of the internal conductor-coil
2 (the ratio of the length of the exposed portions 2a and 2b of the
internal conductor-coil 2 to that of the final windings at the ends
thereof (for example, the ratio is about 0.75 when the length of
the exposed portions is about 3/4 of the length of the final
windings)) and the temperature rise in the connected portions of
the external electrodes 4a and 4b with the internal conductor-coil
2.
As shown in the graph shown in FIG. 5, the temperature rise in the
connected portions between the respective external electrodes 4a
and 4b and the internal conductor-coil 2 can be suppressed by
setting the ratio of the exposed portion per winding of the
internal conductor-coil 2 so as to be not smaller than about 0.66
(2/3 windings).
FIG. 6 is a graph showing the relationship between the amount of
offset of the center X of the internal conductor-coil 2 from the
center Y of the end surface 3a or 3b of the molded magnetic body 3
(the ratio of the offset distance to the inner diameter of the
internal conductor-coil 2 (ratio to the inner diameter of the
coil)) and the temperature rise in the external electrodes 4a and
4b.
As shown in FIG. 6, the temperature rise in the external electrodes
4a and 4b is efficiently suppressed by setting the amount of offset
(the ratio to the inner diameter of the coil) to a value not
greater than about 1/2 (0.9 mm) of the inner diameter of the
internal conductor-coil 2.
When a gap is produced between the lower end of the exposed portion
2a (2b) of the internal conductor-coil 2 and the upper end of the
solder fillet 12, applied current flows only through the external
electrodes 4a and 4b at the gap portion thereof, whereby the
temperature rise in the gap portion of the external electrodes 4a
and 4b substantially increases.
A method for manufacturing the above inductor is described
below.
(1) As shown in FIGS. 7 and 8, to manufacture the above inductor, a
mold 24 is prepared, the mold 24 including an upper mold 22
provided with a substantially annular concave portion 21a provided
in the upper mold 22 at an inner surface thereof opposing an end of
the internal conductor-coil 2 to receive at least one portion of a
final winding of wire at the end of the internal conductor-coil 2,
and a lower mold 23 provided with a substantially annular concave
portion 21b provided in the lower mold 23 at the inner surface
thereof opposing the other end of the internal conductor-coil 2 so
as to receive at least one portion of the final winding of wire at
the other end of the internal conductor-coil 2. Each of the
substantially annular concave portions 21a and 21b has a width of
about 0.3 mm and a depth of about 0.2 mm. However, the shape and
the size of the substantially annular concave portions 21a and 21b
are not limited to those described above, and they may be any shape
and size as long as the internal conductor-coil 2 insulated by a
coating material is received and affixed in the concave portions
21a and 21b.
The mold 24 prevents deformation of the internal conductor-coil 2
(see FIG. 8), and is configured such that a substantially
cylindrical coil-supporting member (protection pin) 25 for
supporting and affixing the internal conductor-coil 2 inside the
mold 24 at a center thereof can be mounted in the mold 24. The
coil-supporting member 25 is mounted substantially at a central
portion of the mold 24 such that the coil-supporting member 25 is
placed on the lower mold 23, and the upper mold 22 covers the lower
mold 23 holding the coil-supporting member 25.
The upper mold 22 is provided with gates 22a and 22b at a side and
an upper portion, respectively, of the upper mold 22, through which
the magnetic material 1 is injected into the mold 24 (see FIGS. 9
and 11).
The mold 24 is configured such that centers of the above annular
concave portions 21a and 21b are positioned substantially at
centers of an inner lower surface 32 of the upper mold 22 and an
inner upper surface 33 of the lower mold 23, respectively.
(2) After the coil-supporting member 25 positioned in the lower
mold 23, the internal conductor-coil 2 is fitted into the
coil-supporting member 25, and the upper mold 22 is positioned on
the lower mold 23 holding the coil-supporting member 25 and the
internal conductor-coil 2, whereby the internal conductor-coil 2 is
supported in a desired position in the mold 24, as shown in FIG. 8,
such that it is not deformed.
(3) As shown in FIG. 9, the magnetic material 1, which is formed by
melting a pellet-formed ferrite resin made by kneading a PPS
(polyphenylene sulfide) resin and a powdered ferrite including iron
oxide (Fe.sub.2 O.sub.3), nickel oxide (NiO), copper oxide (CuO),
and zinc oxide (ZnO), is injected (a first injection) via the gate
22a provided at the side of the upper mold 22 into a region in the
mold 24 except for the inside of the internal conductor-coil 2 (a
region occupied by the coil-supporting member 25).
(4) The coil-supporting member 25 is removed from the mold 24, as
shown in FIG. 10.
(5) The magnetic material 1 is injected (a second injection) via
the gate 22b provided at the upper surface of the upper mold 22
into the inside of the internal conductor-coil 2, whereby the
molded magnetic body (a ferrite-resin-molded body including a coil)
3 having dimensions of, for example, approximately
4.5.times.3.2.times.3.2 (mm) is obtained.
In this case, the temperature in the mold 24 is set at 160.degree.
C., and the temperature of a cylinder for supplying the magnetic
material 1 is set at 340.degree. C.
(6) The molded magnetic body 3 thus obtained is rinsed with pure
water, is well rinsed with alcohol, is deoxidized by applying
palladium solution, and the overall molded magnetic body 3 is
coated with a nickel film, which has a thickness of about 1 .mu.m
to about 2 .mu.m, formed by electroless nickel-plating.
(7) A resist film having a thickness of approximately 10 .mu.m is
printed in a portion to be provided with the external electrodes 4a
and 4b at the ends of the molded magnetic body 3, and is dried at
about 150.degree. C. for 10 about minutes. The molded magnetic body
3 printed with the resist film is dipped for several minutes in a
solution of nitric acid of 30%, thereby removing by etching the
nickel film formed by electroless nickel-plating from a portion
other than the portion corresponding to the external electrodes 4a
and 4b.
(8) The resist film is removed by dipping the molded magnetic body
3 in a solution of sodium hydroxide of about 3% while supersonic
vibration is applied to the molded magnetic body 3.
(9) The molded magnetic body 3 provided with a nickel film formed
by electroless nickel-plating at the ends of the molded magnetic
body 3 is provided with another nickel film having a thickness of
about 1 .mu.m to about 2 .mu.m formed by electrolytic
nickel-plating performed in a barrel, the molded magnetic body 3
being overlaid with the electrolytic nickel film on the electroless
nickel-plating film. The molded magnetic body 3 is further provided
with a tin-film having a thickness of about 3 .mu.m to about 5
.mu.m formed by electrolytic tin-plating on the electrolytic
nickel-plating film, whereby the surface-mounting-type inductor 10
shown in FIG. 1 is obtained.
In the above manufacturing method, the first injection of the
magnetic material 1 is performed via the gate 22a provided at the
side of the upper mold 22. In FIG. 9, although the magnetic
material 1 flows horizontally (along an arrow A), the internal
conductor-coil 2 is not deformed toward the inside because the
internal conductor-coil 2 is supported and affixed by the
coil-supporting member 25. Consequently, the internal
conductor-coil 2 is supported while being applied with pressure
toward the ends thereof (in directions B (a vertical direction)),
and is fixed to the mold 24 in a manner such that the ends 2a and
2b of the internal conductor-coil 2 engage with the substantially
annular concave portions 21a and 21b, respectively, which are
provided in positions at which the ends 2a and 2b of the internal
conductor-coil 2 come into contact, respectively, with the mold
24.
When the second injection of the magnetic material 1 is performed,
the molded magnetic body 3 has ends of the internal conductor-coil
2, each having approximately one winding length, exposed at the end
surfaces 3a and 3b, respectively, of the molded magnetic body 3,
and the projection amount L of the internal conductor-coil 2 from
each of the end surfaces 3a and 3b of the molded magnetic body 3 is
at least about 1/3 of the diameter D of the wire of the internal
conductor-coil 2. As a result, inductor having highly reliable
connectivity is produced, which has a large area of connected
portions between the external electrodes 4a and 4b and the internal
conductor-coil 2, as shown in FIG. 1.
In FIG. 2, the respective centers X of the annular concave portions
21a and 21b substantially coincide with the centers Y of the lower
surface 32 of the upper mold 22 and the upper surface of the lower
mold 33, respectively, whereby the molded magnetic body 3 is
produced in which the centers X of the final winding portions of
the internal conductor-coil 2 substantially coincide with the
centers Y of the end surfaces 3a and 3b, respectively, of the
molded magnetic body 3, as shown in FIG. 1.
Therefore, a risk of phenomena described below is efficiently
avoided. That is, when an inductor in which a coil is displaced is
mounted on a printed circuit board or other suitable component, a
solder fillet does not extend to an exposed portion of the coil
with external electrodes therebetween because the exposed portion
of the coil is excessively elevated, and a gap is produced between
a lower end of the exposed portion and an upper end of the solder
fillet. Therefore, when the external electrodes are made of a
metallic thin film such as a solder film, the long-term reliability
is reduced and safety when an overcurrent is applied is reduced,
due to insufficient current capacity in the portions corresponding
to the gap.
The ratio of an exposed portion in a final winding of wire of an
internal conductor-coil at each end of a molded magnetic body (the
ratio of the exposed portion in a final winding of the internal
conductor-coil of which the projection amount is at least about 1/5
of the diameter of the wire of the internal conductor-coil) and the
amount of offset of the center of the final winding of the internal
conductor-coil from the center of each end surface of the molded
magnetic body were measured for 1000 inductors (samples)
manufactured by the method described above, and the result is shown
in table 1.
TABLE 1 Inductors Conventional according to Items Criteria
Inductors the invention Ratio of exposed 2/3 or more 0.1% 100%
portion per final 1/2 or more 3% 100% winding of 1/3 or more 10%
100% internal coil (projected by at least about 1/5 of wire
diameter) Amount of offset 1/4 or less 0.5% 100% of internal coil
1/3 or less 43% 100% (ratio to inner 1/2 or less 78% 100% diameter
of coil)
In table 1, the ratio of an exposed portion in a final winding of
wire of an internal conductor-coil at each end of a molded magnetic
body (the ratio of the exposed portion in the final winding of wire
of the internal conductor-coil of which the projection amount is at
least about 1/5 of the diameter of the wire of the internal
conductor-coil) and the amount of offset of the center of the final
winding of the internal conductor-coil from the center of each end
face of the molded magnetic body are also shown, which were
measured for 1000 inductors manufactured by a conventional
method.
In table 1, the proportion of the samples (inductors), which met
with the criteria, to 1000 samples are shown.
It is seen from table 1 that the ratio of the inductors
manufactured by the conventional method, of which at least about
2/3 of a final winding of wire project by an amount of at least
about 1/5 of the wire, is only 0.1%, and the ratio of the
inductors, which have the same criteria, manufactured by the method
according to the present preferred embodiment is 100%. Therefore,
according to preferred embodiments of the present invention,
long-term reliability and safety when applied with an overcurrent
are greatly improved by increasing the area of connection between
the internal conductor-coil and the external electrodes.
It is also seen from table 1 that the ratio of the inductors
manufactured by the conventional method, which have the offset
amount of the center of a final winding of wire of the internal
conductor-coil from the center of each end surface of the molded
magnetic body of not greater than about 1/2 of the inner diameter
of the internal conductor coil, is only 78%, and that the offset
amount, when manufactured by the method according to the preferred
embodiments of the present invention, is reduced to be not greater
than about 1/4 of the inner diameter of the internal
conductor-coil.
The present invention is not limited to the above-described
preferred embodiment, and it is intended to include various
arrangements and modifications, within the spirit and scope of the
present invention, regarding the type of the magnetic molding
compound, the particular shape of the molded magnetic body, the
material for the internal conductor-coil, the material for the
baked external electrodes, and other features of the present
invention.
While preferred embodiments of the invention have been disclosed,
various modes of carrying out the principles disclosed herein are
contemplated as being within the scope of the following claims.
Therefore, it is understood that the scope of the invention is not
to be limited except as otherwise set forth in the claims.
* * * * *